1. Field of the Invention
The present invention relates to a W-CDMA (Wideband CDMA) WLL system, and more particularly to a DTMF transmission structure of a W-CDMA WLL system.
2. Description of the Background Art
Generally, in a conventional communication system in which voice is coded by using a Waveform Coding-based PCM, a DTMF signal is transmitted through the same transmission structure as that of a voice signal.
Accordingly, in view of the characteristics of the waveform coding method, even though an original signal is coded or decoded, there occurs little spectrum distortion, so that, even in case of the DTMF signal, it can be transmitted without causing a problem.
However, in a communication system using a parameter/hybrid coding-based VOCODER (a voice coder/decoder), the characteristics of the vocoder is optimized for a voice signal. Thus, when a DTMF signal is transmitted therethrough, there may occur a spectrum distortion in its original signal, so that there is a high possibility that a receiving party including a switching terminal recognizes the DTMF signal erroneously.
In an effort to solve the problem, a commercially used PCS system adopts a method in which when a DTMF signal is transmitted, a input digit value is stored, and the stored information is transmitted by a different transmission method to that of a voice signal, and then the corresponding DTMF signal is reproduced for a predetermined time and transmitted to a switching terminal.
However, in case of the W-CDMA (Wideband CDMA) WLL system which mixedly uses the PCM method and the VOCODER method, since no transmission structure has been defined for the DTMF signal, the DTMF signal is transmitted through the general transmission structure for the voice signal.
Figure is a DTMF transmission structure adopting the W-CDMA WLL system in accordance with a conventional art.
As shown in the drawing, in the conventional DTMF transmission structure, the DTMF signal is transmitted between a terminal 10 and a switching system 40 through a terminal station 20 and a base station controller. The terminal station 20 and the base station controller 30 includes an adder 12, a VOCODER 13 and an echo canceller 14, respectively, and a hybrid unit 11 is connected to an input terminal of the adder 12 at the side of the terminal 10.
The hybrid unit 11 serves to match a 2-wire analog duplex signal (a voice signal or a DTMF signal) inputted/outputted to/from the terminal 10 to a 4-wire analog simplex signal. The adder 12 serves to remove an echo component provided from an echo canceller 15, from the voice signal outputted from the hybrid unit 11. In this case, the input signal and the echo component have the opposite phase to each other, and the signal between the hybrid unit 11 and the VOCODER 13 is a digital signal.
As an element to code or decode an analog voice signal to a digital signal (bit stream) having a low transfer rate, the VOCODER 13 codes a voice signal outputted from the adder 12 to a bit stream form or decodes a signal to a bit stream form inputted through a CDMA modem unit (not shown) to a voice signal.
The operation for transmitting the DTMF signal in the conventional DTMF transmission structure constructed as described above will now be explained.
First, the DTMF signal is transmitted from the terminal 10 to the switching system 40 or from the switching system 40 to the terminal 10. For the explanation""s convenience, the case of DTMF signal transmitted from the terminal 10 to the switching system 40 is taken as an example.
When a user dials a specific number of a target terminal by using the terminal 10 connected to the terminal station 20, the dialed digits, that is, the DTMF signal of the corresponding dial is inputted through the hybrid unit 11 to the adder 12.
However, a leakage current is generated in the hybrid unit 11 due to an impedance matching physically existing therein to match the 2-wire duplex signal and the 4-wire simplex signal, so that an echo phenomenon occurs when the voice signal is transmitted.
Accordingly, in order to cancel the echo component of the output signal of the hybrid unit 11, the echo canceller 14 extracts an echo component from the VOCODER 13 and provides it to the adder 12. Then, the adder 12 subtracts the output signal of the echo canceller 14 from the output signal of the hybrid unit 11, thereby canceling the echo component included in the DTMF signal.
The DTMF signal with the echo component removed is coded to a bit stream form by the VOCODER 13, and then transmitted through the CDMA modem unit and the RF unit to the base station. The RF signal transmitted to the base station is demodulated to a bit stream form by the RF unit and the CDMA modem unit, and the demodulated signal is decoded to a DTMF signal in the VOCODER 13 of the base station controller 30 and transmitted to the switching system 40.
As stated above, since a transmission structure for the DTMF signal is not defined in the W-CDMA WLL system, the DTMF signal is transmitted through the same transmission structure as that of the general voice signal.
However, the VOCODER is fabricated suitable to code or decode a voice signal. Thus, in case that the DTMF signal is coded or decoded by using the VOCODER, a frequency spectrum of the DTMF signal is distorted
In addition, in the conventional transmission structure of the voice signal, the echo component included in the input DTMF signal is forcibly cancelled by the echo canceller. In this respect, in case of the voice signal, the echo cancelling does not cause a problem, but, in case of the DTMF signal, the echo cancelling causes a spectrum distortion. Moreover, such spectrum distortion caused due to the echo cancelling becomes more seriously made as the DTMF signal passes through the VOCODERs of the terminal station 20 and of the base station controller 30.
Accordingly, the spectrum distortion of the DTMF signal due to the echo cancelling and the characteristic of the VOCODER brings about a malfunction in various systems required for signal transmission in the DTMF signal form as well as in the switching terminal, having a bad influence on a reliability of the systems.
Therefore, an object of the present invention is to provide a DTMF transmission structure of a W-CDMA WLL system which is capable of transmitting a high-quality DTMF signal to a receiving party by using an existing voice signal transmission structure.
Another object of the present invention is to provide a DTMF transmission structure of a W-CDMA WLL system which is capable of effectively preventing a spectrum distortion of a DTMF signal caused due to a VOCODER characteristic and a echo cancelling.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a DTMF transmission structure of a W-CDMA WLL system having a terminal station and a base station controller which are formed symmetrically, each including: an echo canceller for extracting an echo component from a received signal; a DTMF detector for discriminating whether a signal to be transmitted is a DTMF signal and controlling the operation of the echo canceller; an adder for canceling an echo component of the echo canceller from the signal to be transmitted; a VOCODER for coding the output signal of the adder to a bit stream form or decoding the received signal in a bit stream to an analog signal; an adjusted DTMF detector for detecting a digit component from the output of the VOCODER; A DTMF generator for reproducing a normal DTMF signal according to the detected digit component; and a silence generator for generating a silence for a predetermined time immediately after the DTMF is generated.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.